This is a description taken from LEGO.com. Please do not modify it. (visit this item's product page)RF ID Sensor for your MINDSTORMS® NXT robots!
Another great MINDSTORMS NXT accessory for experienced programmers! Add an NXT-G program loop that needs a specific transponder to run. When you have the key transponder, only you can start the program – use multiple transponders to run multiple program paths!

Includes sensor and 2 RF transponders!

Does not include connector cable.

This is the individual component, and does not include the NXT Intelligent Brick, software or any accessories. It requires the complete LEGO® MINDSTORMS® NXT set (#8527) for use.

Another great MINDSTORMS NXT accessory for experienced programmers! Add an NXT-G program loop that needs a specific transponder to run. When you have the key transponder, only you can start the program – use multiple transponders to run multiple program paths!

Includes sensor and 2 RF transponders!

Does not include connector cable.

This is the individual component, and does not include the NXT Intelligent Brick, software or any accessories. It requires the complete LEGO® MINDSTORMS® NXT set (#8527) for use.